Message-Id: <200609271840.32874.david-b@pacbell.net>
Date:	Wed, 27 Sep 2006 18:40:32 -0700
From:	David Brownell <david-b@...bell.net>
To:	tglx@...utronix.de
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>, mingo@...hat.com
Subject: Re: [Bulk] Re: [Bulk] Re: [patch 2.6.18] genirq: remove oops with fasteoi irq_chip descriptors

Hi Thomas,

On Wednesday 27 September 2006 6:18 pm, Thomas Gleixner wrote:
> Dave,
> 
> On Wed, 2006-09-27 at 17:39 -0700, David Brownell wrote:
> >  - It wouldn't use chip->mask_ack() when that exists and those
> >    other two routines don't, even though mask_ack_irq() is a
> >    conveniently defined inline.
> 
> So why not replace it by mask_ack_irq() ?

It'd still oops on chips with just enable(), disable(), and eoi().

Which, on my brief scan of the codebase, appears to be one of the
accepted ways to craft a fasteoi irq_chip.


> >  - Umm, how could it ever be correct to leave the IRQ active
> >    without a dispatcher?  ISTR the rationale for that delayed
> >    disable was not purely to be a PITA for all driver writers,
> >    but was to address some issue with edge triggering.  In that
> >    path, triggering was no longer to be allowed ...
> 
> Your patch would result in default_disable() when no shutdown function
> is provided. default_disable() does the delayed disable thing, while you
> remove the handler. The next event on that line will cause a spurious
> IRQ.

That may be an argument that the default shutdown() should not be the
same as the default disable().  Unless shutdown() is going away??

I still dislike that delayed disable() mechanism.  Every time I've
seen ("tripped over") it in action it's been the cause of bugs.

 
> >  - Plus ack()ing the IRQ there just seemed pretty dubious.  It's
> >    not like there would be anything preventing that signal line
> >    from being lowered (or raised, etc) immediately after the ack(),
> >    which in some hardware would latch the IRQ until later unmask().
> >
> > Leaving the question:  what's the point of it??  The overall
> > system has to behave sanely with or without the ack(); just
> > clearing a latch doesn't mean it couldn't get set later.
> 
> Fair enough.
> 
> > > > So what's the correct fix then ... use enable() and disable()?
> > > > Oopsing isn't OK... 
> > > 
> > > True, but we can not unconditionally change the semantics. 
> > 
> > Some current semantics are "it oopses".  That's a good definition
> > of semantics that _must_ be changed.  We're not Microsoft.  ;)
> 
> Agreed, it just depends on how they get fixed.

I thought maybe submitting a reasonably sane patch would be the
best way to start that discussion.  :)

The only issue appears to be how that rarely-used "get rid of
the handler" code path should work.

 
> > > Does it break existing or new code ?
> > 
> > Could any code relying on those previous semantics have been
> > correct in the first place, though?  Seemed to me it couldn't
> > have been.
> > 
> > Plus, unregistering IRQ dispatchers is a strange notion.  I've
> > never seen it done in practice ... normally, they get set up once
> > during chip/board setup then never changed.  Bugs in code paths
> > like that have been known to last for decades unfixed.
> 
> Agreed. Nothing is using this currently.

Aha!  So if it's "nothing" then that rarely/not-used path can change
without negative impact...


> > > Sorry, I did not think about the defaults in the first place. The
> > > conditionals in manage,c are probably superflous leftovers from one of
> > > the evolvement.
> > 
> > And that's how I was taking that particular mask() then ack() too,
> > especially given it never used mask_ack() when it should have, and
> > since that logic oopsed in various cases with fasteoi handlers.
> 
> The remaining question is whether mask_ack_irq() or shutdown() is the
> correct approach. Your patch would make it mandatory to implement
> shutdown at least for such removable stuff.

Well, an implementation of shutdown() _is_ always provided.  At least
now; I don't have time to track your MM patches.


> I'm not sure about that right now as I'm too tired.

I expect that after you sleep on this, something will come to mind.  ;)

- Dave


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists